Ultrasonic meters (USM) have become increasingly popular for fiscal flow metering worldwide. Ultrasonic meters are desirable because they can measure a wide range of flow rates with excellent accuracy, they have less impact on pressure drops, and they have no moving parts.
One element of a USM is an ultrasonic transducer, which transmits ultrasonic signals to and receives ultrasonic signals traveling through substances flowing in a pipe (or spool). Such pipes are usually made of steel. As a result, the joint interface between the pipe and transducer mounting holding the bracket is vulnerable to rust. To avoid rust, a common rubber gasket can be used. However, common rubber gaskets cause signal back-coupling especially in low pressure applications. Unwanted back-coupling resulting from the acoustic vibration and/or oscillation of the mounting bracket and/or of meter body as well as from the returned signals from backward propagation through bracket and meter body deteriorate signal quality and obscure signal detection, causing unstable performance of the meter.
To avoid such undesirable back-coupling, a small air gap between the meter body and holding bracket can be used to insulate the ultrasounds. But the air gap is vulnerable to moisture and temperature variations, which also causes rust. Whenever rust is formed, it in turn can create back-coupling and reduce meter performance.
The oil and gas industry has safety regulations for hazardous and flammable zones, along with regulatory restrictions regarding metering accuracy. These rules require USM's to provide a signal that is robust and that has a good signal-to-noise ratio. In addition, USM's must meet certain sensitivity metrics in order to work in various environmental conditions. These standards are difficult to meet given the numerous disturbances that can be present. Rusted parts are not only a durability concern, but also induce back-coupling, which results in meter performance problems.
While rusting is a separate issue from back-coupling, they are actually interrelated problems, particularly in meter body and transducer performance. First, transducers need to be mounted inside the spool. The joint interface must be completely sealed and gas tight. Although a rubber gasket might work to seal out moisture and vapor and dust, such a seal requires an extremely tight fit. However, the tighter the joint interface gets, the more severe the unwanted back-coupling becomes. Thus, the use of traditional rubber gaskets is untenable for mounting USM transducers.
Accordingly, methods and systems for improved ultrasonic meter transducers are required as disclosed herein.